1. Field
This patent specification relates generally to image processing, and more specifically to systems and methods for image processing by improved color image signal quantization and compression, and computer accessible storage media for implementing such processing.
2. Discussion of the Background
As the use of color imaging apparatuses becomes more widespread, it becomes increasingly important to improve techniques capable of suitably handling image data and providing color images which are reproduced with deviation as small as possible with respect to original images.
In the transmission and storage of the image data, it is known in general to provide encoding (or quantization) and compression of color image data in order to reduce the data size to be processed.
It is also known in the data processing that RGB components for the primary colors are correlated highly each other. Prior to the image data compression, therefore, the image data are subjected to the color transformation into less correlated components such as lightness and color signals.
Several methods have been adopted for the color image signal transformation such as, for example, the YCbCr transform which carries out the transformation from RGB components into brightness signals Y and color signals Cb and Cr, utilizing the transformation matrix
                              [                                                    Y                                                                    Cb                                                                    Cr                                              ]                =                                            [                                                                    0.29                                                                                                                                                  ⁢                      0.587                                                                            0.114                                                                                        0.1687                                                                              -                      0.3313                                                                            0.5                                                                                        0.5                                                                              -                      0.4187                                                                            0.0813                                                              ]                        ⁢                                                  [                                                            R                                                                              G                                                                              B                                                      ]                    .                                    (        1        )            
The reversible component transform (RCT) may also be used alternatively as a simpler and faster method, in which RGB components are transformed into brightness signals Y and color signals U and V, utilizing the transformation matrix
                    {                                                                              Y                  =                                      ⌊                                                                  (                                                  R                          +                                                      2                            ⁢                            G                                                    +                          B                                                )                                            /                      4                                        ⌋                                                                                                                        U                  =                                      R                    -                    G                                                                                                                        V                  =                                      B                    -                    G                                                                                ,                                    (        2        )            where the notation └ ┘ is for representing the floor function.
Incidentally, since the above noted transform operations are obviously reversible, RGB components can be reproduced by performing the reverse transform on the brightness signals Y and color signals U and V.
The RCT transformation method is implemented in general to decorrelate the RGB components in pixels, thereby improving the rate of data compression, and has the form as simple as possible placing priority on higher speeds of data processing.
This results in a non-uniform color space such as those exemplified by YUV after the color transformation, in which the magnitude of the variation in the brightness signals Y and color signals U and V, in general, does not correspond linearly to that of human color perception. This may also result in a decrease in color image quality, that is recognized when color component are quantized linearly, especially with a higher degree of data compression.
In previous methods, however, color signal transformation have been carried out in practice to achieve the quantization with more ease (for example, quantizing linearly) rather than utilizing the original RCT capability of the high rate of data compression. Namely, color signals are subjected to the transformation into a uniform color space such as, for example, the Lab space (CIE 1976) with linear color characteristics, and the color space is then divided in uniform into a plurality of quantization regions, according the pervious methods.
As an example, U.S. Pat. No. 5,307,088 describes a method of first transforming RGB components into the Lab space and then quantizing or encoding the resultant signals in this space, in a similar manner to the above noted methods of transforming into a uniform color space and subsequently dividing in uniform into multiple quantization regions.
Also, U.S. Pat. No. 5,072,290 describes a method of quantizing and encoding in the Lab space, in which further means of referencing the variable L is proposed, since the Lab space is not a uniform color space to be exact.
These methods are both designed such that the color signals be transformed into a uniform color space, and that quantization or encoding of color components is carried out in consideration primarily of the ease of computation, rather than utilizing the characteristics of the color space, in which high speed transformation can be achieved. That is, the ease of the quantization steps is regarded highly at the expense of the speed.
However, since a relatively large amount of computation is required in general for the transformation into the Lab space, the above described methods are not entirely suitable for the high speed computation for the color signal transformation. Moreover, when additional steps for the quantization are included such as referring to L as the variable, as described above, this may give rise to a considerable decrease in speed of the signal transformation.